Metal-air batteries are potentially the least expensive primary batteries available, typically have a very high energy density and are environmentally benign. They have the highest gravimetric and volumetric energy density of any primary battery system. Recharging of these batteries is relatively ineffective. Developmental rechargeable metal-air batteries have a life of only a few hundred cycles and an efficiency of only about 50%. However, metal-air batteries can be refueled, often referred to as a metal-air fuel cell when configured for refueling, where the consumed metal is mechanically replaced and the spent metal oxide can be reduced to the metal in a separate device. The term metal-air fuel cell is also used in reference to an electro-chemical system where the metal is used as a co-reactant to assist the reformation of an alternate fuel, for example hydrogen, a hydrocarbon, or an alcohol. As such a “metal” economy where vehicles and portable equipment and devices can be powered by these metal-air batteries or metal-air fuel cells has been envisioned.
Research is ongoing for electric vehicles using metal-air batteries, especially zinc-air batteries as zinc is inexpensive and readily mass produced. These batteries have specific energies up to 370 W·h/kg whose terminal voltage does not drop until 80-85% depletion of the metal. Such batteries have very long shelf lives when sealed to exclude oxygen but having very high self-discharge rates when exposed to air. Small zinc-air batteries are commonly used for hearing aids, and a line of very thin zinc-air batteries are being introduced in mid-2009 for use as low cost long-life primary batteries for consumer electronic devices.
The anodes of metal-air batteries are commonly metals in a particulate form mixed with an electrolyte, such as a hydroxide, in the form of a paste that release electrons upon oxidation. The air electrodes are typically made of porous carbon structures on metal meshes that are covered with oxygen reduction catalysts forming hydroxide by the reduction of oxygen and its subsequent reaction with water. Where the metal is zinc, the reaction has a potential to produce a maximum of 1.65 V, which is typically reduced to 1.35 to 1.4 V by limiting the air flow into the cell.
Air cathodes used in polymer electrolyte membrane (PEM) fuel cells typically contain metals, in particular precious metals such as platinum. These cathodes can work well, but they are typically very expensive. Improvements in metal-air batteries and fuel cells are generally tied to improvements in the air cathodes.
Nearly all air cathodes are a typically sheet-like members having opposite surfaces exposed to the atmosphere and to the aqueous electrolyte of the cell. The air cathode must be permeable to air or another source of oxygen, but must be substantially hydrophobic so that aqueous electrolyte will not seep or leak through it and has an electrically conductive element connected to external circuitry. The construction of conventional air cathodes is described by Bidault et al, “Review of gas diffusion cathodes for alkaline fuel cells” Journal of Power Sources, 187 (2009) 39-48. Generally they comprise a thick film, having multiple layers of nanoscale metal catalyst impregnated active carbon that are mixed with polytetrafluoroethylene (PTFE) particles affixed to an electrically conducting backing layer. They achieve a high areal oxygen reduction capability because of a large three phase interface between the meandering hydrophobic PTFE particles and the electrolyte wetted carbon supported catalyst.
These conventional air cathodes often display several shortcomings. As few of the pathways are purely hydrophobic, containing a mixture of hydrophilic catalyst and carbon, pores constructed to provide pathways for gas phase oxygen penetration to the catal yst particles can flood with electrolyte. Flooding greatly slows the diffusion of oxygen to the catalyst surfaces. Prevention of flooding requires that the pressure and humidity of the oxygen source be carefully controlled. A second shortcoming is a kinetic barrier to diffusion of the hydroxide ions because of the tortuous pathway for ion diffusion through the hydrophilic portion of the cathode that generates current limiting impedance. The metal catalysts are often a precious metal, such as platinum, which renders the cathode relatively expensive. A major fraction of the precious metal used in fuel cells, hence its cost, lies in the cathode.
For hydrocarbon fuel cells, such as direct methanol fuel cells, methanol cross over from the anode to the cathode is a major issue. Power generation efficiency of the direct methanol fuel cell significantly decreases when methanol or methanol oxidation products reach the cathode side of the cell. Proton exchange membranes, such as Nafion® membranes, are currently used in many fuel cell to significantly reduce, rather than to prevent, the methanol cross over from anode to cathode while maintaining the proton conduction, as reviewed by Arico et al, in “DMFCs: from fundamental aspects of technology development” Fuel Cells 1, (2001) 133-161. In Pt-containing conventional air cathodes, the nanoscale metal particles are subject to migration, agglomeration and particle growth by Oswald ripening. Since agglomeration and particle growth reduce the surface area of the catalytic sites, the efficiency of the cathode degrades over time. Precious metal catalysts are also subject to poisoning by carbon monoxide, which can form during fuel oxidation, or from impurities in the oxygen source and/or fuel.
Hence an air cathode that avoids precious metals, is thin, resists flooding, has little or no hydroxide ion diffusion barrier, and is not poisoned by CO would constitute a significant improvement in technologies that employ them.